1. Field of the Invention
The present invention relates to copolyimides. In particular, it relates to soluble copolyimides prepared from 4,4'-oxydiphthalic anhydride, 3,4,3',4'-biphenyltetracarboxylic dianhydride and 3,4'-oxydianiline.
2. Description of the Related Art
Aromatic, thermoplastic polyimides are a class of polymers used in a variety of high performance/high temperature applications. Such applications include adhesives, matrix resins for composites, and high strength films and coatings. These thermoplastic polyimides are usually soluble in either high boiling polar protic phenolic solvents, such as m-cresol and chlorophenol or halogenated solvents such as tetrachloroethylene and multihalogenated aromatics, many of which are highly toxic and are not used in large scale industrial processes without solvent recovery systems. Thus, the majority of polyimide thermoplastics are solution processed in the polyamic acid state using milder solvents and subsequently cyclodehydrated to form the final polyimide article. However, there is a disadvantage to using the polyamic acid intermediates in that they are unstable, susceptible to hydrolysis, and generate water during imidization.
Attempts to increase the solubility of polyimides generally involve the modification of the polymer backbone through monomer selection. These modifications include the incorporation of flexible aliphatic units, polar and nonpolar pendent substituents, heteroatoms or groups, and polyimide copolymers which contain either mixtures of the above monomers with common aromatic diamines and dianhydrides, or block segments of soluble oligomers. Although most of these polyimides meet and even exceed some of the criteria required to find wide spread use as high performance materials, cost limits their acceptance.
By the present invention, wholly aromatic, thermoplastic polyimide copolymers were prepared based on 4,4'-oxydiphthalic anhydride, 3,4,3',4'-biphenyltetracarboxylic dianhydride and 3,4'-oxydianiline. These copolyimides were found to be tough thermoplastics which are soluble in common amide solvents such as N,N'-dimethyl acetamide (DMAc), N-methylpyrrolidinone (NMP), and dimethyl formamide (DMF) and thus can be applied as the fully imidized copolymer in addition to the amic acid solution.
Meterko et al. (U.S. Pat. No. 5,171,828) had prepared copolyimides from 4,4'-oxydiphthalic anhydride, 3,4,3',4'-biphenyltetracarboxylic dianhydride and 4,4'-oxydianiline or para-phenylenediamine. The difference between the present invention and that of Meterko et al. lies in the use of 3,4'-oxydianiline as compared to 4,4'-oxydianiline. Meterko et al. prepared polyimide films by pouring the polyamic acid into the desired form. The polyamic acid was then cured either by heating or through chemical imidization. Meterko et al. found that their copolyimides had unexpectedly high comparative tracking indexes which made the copolyimides useful as insulators for various electronic applications. It is important to note that Meterko et al. applied their copolyimide as a polyamic acid prior to imidizing and not as an imidized solution.
St. Clair et al. (U.S. Pat. No. 5,147,966) prepared polyimide molding powders, coatings, adhesives and matrix resins from 3,4'-oxydianiline and 4,4'-oxydiphthalic anhydride. They found that by preparing polyimides and polyamic acids from these monomers, along with suitable endcaps, adhesives, composite matrix resins, neat resin moldings, and films or coatings had identical or superior properties to commercially available polyimides. Unfortunately, as with Meterko et al., these polyimide homopolymers were applied as the amic acid solution.
Tamai et al. (U.S. Pat. No. 5,268,446) prepared readily melt-processable polyimides by reacting 3,3'4,4'-biphenyltetracarboxylic dianhydride with 3,4'-oxydianiline. They found that these polyimides had excellent processability and chemical resistance in addition to essential heat resistance. These polymers may be used in various fields such as electric and electronic appliances, space and aeronautical equipment and transportation machinery. They note in example 1 that the polyimide was insoluble in halogenated hydrocarbon solvents such as methylene chloride and chloroform. In addition, they prepared films from the amic acid solution instead of from the imide powder.
An object of the present invention is to prepare a tough, soluble, aromatic, thermoplastic copolyimide.
Another object of the invention is to prepare a diverse group of articles from the copolyimide.